Vehicle brake systems having electronic pressure regulation are discussed for example in DE 10 2009 001135 A1.
This vehicle brake system includes a first actuator system, hereinafter designated the primary actuator system, in the form of a conventional ABS/ESP brake system for the wheel-individual modulation of the brake pressure as a function of the slippage conditions prevailing at the wheels. The brake pressures of the individual wheel brakes can be set or regulated independently of one another. In connection with a conventional four-wheeled motor vehicle, one therefore speaks of a four-channel regulating actuator system. The brake system includes, inter alia, a pressure medium aggregate made up of a housing block equipped with pumps and valves and an electronic control device that actuates the pumps and valves as a function of sensor signals that describe the slippage conditions at the individual wheels.
This primary actuator system makes it possible to stabilize a driving state of a vehicle during a braking process, during startup, or during driving operation, by braking the wheels affected by slippage in a targeted manner. The brake pressure required for this can be produced together with the driver or independently of the driver. Accordingly, the primary actuator system operates in a so-called partially active mode or in a fully active mode.
In addition, such a vehicle brake system has a second actuator system, or secondary actuator system, in the form of an electromechanical brake booster. This secondary actuator system is connected to the master brake cylinder, and in normal operation is used to increase driver comfort by supporting the driver in the building up of a brake pressure required for braking process. For this purpose, an electromechanical brake booster includes an actuator that can be controlled by an electronic control device, which provides an external force for actuating a master brake cylinder. The actuation of the master brake cylinder can take place solely via the external force of the secondary actuator system, or through a combination of this external force with muscular force provided by the driver.
The first and second actuator system, or primary and secondary actuator system, accordingly form two mutually redundant systems for producing and modulating a brake pressure in a vehicle brake system, where this brake pressure modulation can be carried out with or without the participation of the driver. The two actuator systems thus provide an essential basic precondition for realizing and carrying out partly or fully automated driving operation. Because during such automated driving operation the driver performs only a monitoring function, particularly high demands with regard to safety against failure are made on such vehicle brake systems having electronic pressure regulation, which demands are met by maintaining the mentioned redundancy.
However, the known secondary actuator system, differing from the primary actuator system, is capable of supplying all wheel brakes of the vehicle brake system connected to the master brake cylinder with a uniform brake pressure, or modulating this brake pressure in a uniform fashion, only through actuation of the master brake cylinder. Experts refer to this functioning as one-channel regulation actuator technology. A one-channel secondary actuator system is nonetheless capable of braking a vehicle to a standstill, while maintaining directional stability, in the case of malfunction of the primary actuator system.
Minimum requirements for the longitudinal or directional stabilization of the vehicle during a braking process controlled by the secondary actuator system are: the maintenance of a locking sequence, i.e. a building up of brake pressure in such a way that the wheel brakes of the front axle reach their locking limit temporally before the wheel brakes of the rear axle; maintaining the steerability of the vehicle and consequently ensuring a maximum locking time of the vehicle wheels; and the possibility of an active, or driver-independent, buildup of a brake pressure.
In particular given high deceleration values, the wheels of the rear axle tend to lock before the wheels of the front axle, and can thus bring about an unstable vehicle state. The maximum deceleration values of the vehicle that can be achieved therefore depend strongly on a braking power that can be realized by the wheel brakes of the rear axle. However, this braking power is relatively low due to the dynamic axle load displacement in the direction of the front axle that takes place during a braking process for reasons of mass inertia, because an increase in the axle load on the front axle necessarily entails a reduction in the axle load on the rear axle.
Due to the explained property of the secondary actuator system of being able to bring about only a uniform brake pressure at all wheel brakes that are present, in combination with a low brake pressure that can be realized by the wheel brakes of the rear axle without danger of locking of the associated wheels, in the case of a braking process in which the brake pressure is produced by the secondary actuator system due to an occurrent malfunction at the primary actuator system the disadvantage results that an overall braking power that can be realized of the vehicle turns out to be relatively low, or that consequently a relatively long brake path of the vehicle results. This has a particularly negative effect in vehicles in which the dynamic axle load displacement in the direction of the front axle during a braking process is particularly large.
A known approach for avoiding this disadvantage is to adapt the controlling of the secondary actuator system to the mentioned dynamic axle load displacement in the direction of the front axle, in combination with a decoupling of the wheel brakes of the rear axle from the wheel brakes of the front axle with regard to the brake pressure that is present. As a consequence, the brake pressure provided by the secondary actuator system can still be converted to its full extent into braking power by the more strongly loaded wheels of the front axle, but is nonetheless higher than a braking power that can be realized by the wheel brakes, correspondingly relieved of load, of the rear axle. Under these conditions, in order to prevent overbraking of the rear axle, or a locking of the wheels of the rear axle, and consequently an unstable vehicle state, the rear axle is decoupled from the front axle with regard to its brake pressure.
This decoupling takes place with the aid of a valve device that reduces the brake pressure at the wheel brakes of the rear axle compared to the brake pressure at the wheel brakes of the front axle. This reduction is carried out to a brake pressure level that can be completely converted by the wheel brakes of the rear axle into a braking power without the locking of one of the associated wheels of the rear axle.
Due to the occurrent malfunction of the primary actuator system, its valve devices cannot be controlled, and therefore cannot easily be used for this purpose. To separately provide for this purpose an additional valve device, including an additional electronic control device for controlling, would require a large outlay and would be expensive, and would require additional constructive space in the already-tight space conditions.